Re: [PATCH v2 1/6] KVM: arm64: Introduce helper to retrieve a PTE and its level

[Marc Zyngier <maz@kernel.org>]

Hi Alex,

On Tue, 27 Jul 2021 16:25:34 +0100,
Alexandru Elisei <alexandru.elisei@arm.com> wrote:
> 
> Hi Marc,
> 
> On 7/26/21 4:35 PM, Marc Zyngier wrote:
> > It is becoming a common need to fetch the PTE for a given address
> > together with its level. Add such a helper.
> >
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/include/asm/kvm_pgtable.h | 19 ++++++++++++++
> >  arch/arm64/kvm/hyp/pgtable.c         | 39 ++++++++++++++++++++++++++++
> >  2 files changed, 58 insertions(+)
> >
> > diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
> > index f004c0115d89..082b9d65f40b 100644
> > --- a/arch/arm64/include/asm/kvm_pgtable.h
> > +++ b/arch/arm64/include/asm/kvm_pgtable.h
> > @@ -432,6 +432,25 @@ int kvm_pgtable_stage2_flush(struct kvm_pgtable *pgt, u64 addr, u64 size);
> >  int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
> >  		     struct kvm_pgtable_walker *walker);
> >  
> > +/**
> > + * kvm_pgtable_get_leaf() - Walk a page-table and retrieve the leaf entry
> > + *			    with its level.
> > + * @pgt:	Page-table structure initialised by kvm_pgtable_*_init().
> 
> Yet in the next patch you use a struct kvm_pgtable_pgt not
> initialized by any of the kvm_pgtable_*_init() functions. It doesn't
> hurt correctness, but it might confuse potential users of this
> function.

Fair enough. I'll add something like "[...] or any similar initialisation".

> 
> > + * @addr:	Input address for the start of the walk.
> > + * @ptep:	Pointer to storage for the retrieved PTE.
> > + * @level:	Pointer to storage for the level of the retrieved PTE.
> > + *
> > + * The offset of @addr within a page is ignored.
> > + *
> > + * The walker will walk the page-table entries corresponding to the input
> > + * address specified, retrieving the leaf corresponding to this address.
> > + * Invalid entries are treated as leaf entries.
> > + *
> > + * Return: 0 on success, negative error code on failure.
> > + */
> > +int kvm_pgtable_get_leaf(struct kvm_pgtable *pgt, u64 addr,
> > +			 kvm_pte_t *ptep, u32 *level);
> > +
> >  /**
> >   * kvm_pgtable_stage2_find_range() - Find a range of Intermediate Physical
> >   *				     Addresses with compatible permission
> > diff --git a/arch/arm64/kvm/hyp/pgtable.c b/arch/arm64/kvm/hyp/pgtable.c
> > index 05321f4165e3..78f36bd5df6c 100644
> > --- a/arch/arm64/kvm/hyp/pgtable.c
> > +++ b/arch/arm64/kvm/hyp/pgtable.c
> > @@ -326,6 +326,45 @@ int kvm_pgtable_walk(struct kvm_pgtable *pgt, u64 addr, u64 size,
> >  	return _kvm_pgtable_walk(&walk_data);
> >  }
> >  
> > +struct leaf_walk_data {
> > +	kvm_pte_t	pte;
> > +	u32		level;
> > +};
> > +
> > +static int leaf_walker(u64 addr, u64 end, u32 level, kvm_pte_t *ptep,
> > +		       enum kvm_pgtable_walk_flags flag, void * const arg)
> > +{
> > +	struct leaf_walk_data *data = arg;
> > +
> > +	data->pte   = *ptep;
> > +	data->level = level;
> > +
> > +	return 0;
> > +}
> > +
> > +int kvm_pgtable_get_leaf(struct kvm_pgtable *pgt, u64 addr,
> > +			 kvm_pte_t *ptep, u32 *level)
> > +{
> > +	struct leaf_walk_data data;
> > +	struct kvm_pgtable_walker walker = {
> > +		.cb	= leaf_walker,
> > +		.flags	= KVM_PGTABLE_WALK_LEAF,
> > +		.arg	= &data,
> > +	};
> > +	int ret;
> > +
> > +	ret = kvm_pgtable_walk(pgt, ALIGN_DOWN(addr, PAGE_SIZE),
> > +			       PAGE_SIZE, &walker);
> 
> kvm_pgtable_walk() already aligns addr down to PAGE_SIZE, I don't
> think that's needed here. But not harmful either.

It is more that if you don't align it down, the size becomes awkward
to express. Masking is both cheap and readable.

> 
> Otherwise, the patch looks good to me:
> 
> Reviewed-by: Alexandru Elisei <alexandru.elisei@arm.com>

Thanks!

	M.

-- 
Without deviation from the norm, progress is not possible.

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